How do I orient a face so it lays flat?
The transform displayed is the transform of the Object. This particular face has been rotated with respect to object it belongs to, in Edit Mode.

One way to  align the object to its  face, and center it:


In Edit mode, select the face, and in the Header >Transform Orientations menu, hit the little '+' to create a Custom Orientation from the face.
ShiftS > Cursor to Selected
Object Mode, Header > Options menu, check 'Origins'





ShiftS > Selected to Cursor
Header > Object > Transform > Align to Tansform Orientation.


(Uncheck the Origins option)

Now if you set the rotation to 0, the object will align to the world, and its face will follow.